1. Field of the Invention
The present invention relates to an in-situ spectrograph suitable for use in stepper matching during photolithography processes as used in the fabrication of semiconductor devices.
2. State of the Art
Conventionally, semiconductor devices are fabricated in part through the use of optical lithography techniques using projection imaging systems known as steppers. An exemplary projection image system 20 is illustrated in FIG. 1. The projection image system 20 at a minimum includes an illumination controller 22 and an illumination source 24 coupled with and controlled by the illumination controller 22. Illumination source 24 may include a mirror, a lamp, a laser, a light filter, and/or a condenser lens system. As used herein, the term “light” refers to light used in photolithography. The term “light” need not be restricted to visible light, but may also include other forms of radiation suitable for use in lithography. For example, energy supplied by lasers, photons, ion beams, electron beams, or X-rays are included within the term “light.” Excimer lasers and other pulsed radiation sources are typically used in photolithography processes.
Illumination source 24 emits light or radiation that can pass through openings in a reticle 26. The reticle typically comprises a photomask formed on a glass blank. Conventional materials for the blank include soda-lime glass, borosilicate glass or fused silica. The photomask is formed by a film of opaque material. Conventionally, the film is formed with chrome less than 100 nm thick and covered with an anti-reflective coating, such as chrome oxide. The purpose of the anti-reflective coating is to suppress ghost images from the light reflected by the opaque material. The photomask may include a pattern for projecting a wiring or feature pattern of an integrated circuit. The pattern may include various image structures, for example, clear areas, opaque areas, phase shifting areas, and overlay targets. The reticle 26 generally comprises a combination of clear areas and opaque areas, where the clear areas allow light from the illumination source 24 to pass through the mask of the reticle 26 to form the reticle image.
Conventionally, a thin transparent membrane, referred to as a pellicle membrane, is applied over the photomask portion of the reticle to keep the photomask portion free of foreign particles and is usually positioned at a selected height above the photomask. Such height is greater than the focal length of the light imaged onto the photomask. Thus, small particles on the pellicle membrane will not block light from reaching the photomask.
The projection image system 20 shows the reticle 26 positioned adjacent to the illumination source 24; optionally, other devices such as the pellicle or optical lenses may be interposed between the illumination source 24 and the reticle 26. Light passing through the reticle 26 is further transmitted by a projection lens 30, which may be, for example, a reduction lens or a combination of lenses for focusing the reticle pattern onto a projection surface 111. Typical semiconductor fabrication photolithography techniques employ a four- to ten-times reduction of the reticle size by projection lens 30. Projection surface 111 is held in position by a stage, or holding device (not shown), which may be part of or controlled by a stepper (not shown).
In photolithography equipment, a reticle stage may support the reticle and a wafer stage may support the semiconductor wafer, i.e., the work piece being processed. The stages, as well known in the art, typically provide precision motion in the X-axis and Y-axis directions and often some slight provision is made for adjustments in the vertical (Z-axis) direction. A reticle stage is typically used where the reticle is being scanned in a scanning exposure system, to provide smooth and precise scanning motion in one linear direction and insuring accurate, reticle to wafer alignment by controlling small displacement motion perpendicular to the scanning direction and a small amount of “yaw” (rotation) in the X-Y plane. The light may be directed through the reticle and through the stage itself to the underlying projection lens. Thus, the stage itself provides passage for the light.
The projection surface 111 conventionally employed in the manufacture of semiconductor devices typically includes photoresist material disposed over a layer of a semiconductor device structure, such as a wafer or bulk semiconductor material that is to be patterned. The light directed through the reticle may now “expose” and fix portions of the photoresist to define an etch mask pattern in the photoresist.
The photoresist may be developed by removing either the exposed portions of resist for a positive resist or the unexposed portion for a negative resist to form the etch mask. The substrate is subsequently processed as by etching to form the desired structures followed by removal of the photoresist, if desired or required.
As dimensions of features on semiconductor devices continue to decrease, the resolution limits of optical lithography are quickly being reached. One limit is caused by proper positioning of the projection surface in the optimum focal plane, further hampered by insufficient depth of focus. Over the past decade photolithographic systems have evolved through several generations. The wavelength of the illumination source has steadily decreased from 365 nm (i-line of mercury) to 257 nm (high-pressure mercury arc lamp) to 248 nm (KrF laser), and is presently at 193 nm (ArF excimer laser). The numerical aperture (NAo) of the projection lens, having increased from its value of ˜0.16 in the early days to ˜0.86 in present-day systems, is likely to increase still further. Since the resolution limit in the reduction projection exposure method is in proportion to the exposure wavelength and is in inverse proportion to the numerical aperture of the protection lens, resolution improvement has been promoted by the shortening the exposure wavelength and increasing the numerical aperture of the projection lens. On the other hand, the depth of focus of the projection lens is in proportion to the exposure length, and is in inverse proportion to the square of the numerical aperture of the projection lens. By attempting to improve the resolution, therefore, the depth of focus has been abruptly decreased. That is to say, it is difficult to make fine patterns compatible with providing sufficient depth of focus. Especially when high resolution is the goal, the depth of focus becomes very shallow. Therefore, as images are exposed, because of the relatively small dimensional tolerances and high dimensional resolution that are desired of the various structures of semiconductor devices, it is extremely important that the projection surface be as close to the optimum focal plane as possible.
Illumination source settings are a significant factor for optimizing the focal plane, and thus semiconductor device feature dimensions, as the illumination source settings often change from one stepper to another. For example, if the illumination source of a given stepper has different settings than those used with the illumination source of another stepper in the same fabrication process, the light, having different wavelengths, will be refracted differently by the reticle 26 (FIG. 1). The light will therefore enter and exit the projection lens 30 at different locations. Refraction is the bending of a wave resulting from a change in its phase velocity as its moves from one medium to another having a different index of refraction. The index of refraction and, therefore, the bending of the wave, varies with the frequency of radiation (or wavelength) of light.
The light exiting the projection lens 30 at different locations results in a change of the focal plane of the reticle. Such a change of the focal plane due to the different wavelength of the light can result in bad registration of the image onto the wafer, which, in turn, can result in bad overlay from one pattern to another.
The measurement of overlay between successive patterned layers on a wafer is one of the most critical process control techniques used in the manufacturing of integrated circuits and devices. Overlay generally pertains to the determination of how accurately a first patterned layer aligns with respect to a second patterned layer disposed above or below it. A stepper's optical characteristics, such as the illumination source settings and optical aberrations of the lithographic lens system, have a strong influence on overlay and critical dimension performance when patterning advanced design rule integrated circuits. Stepper matching has been utilized to reduce the impact of optical aberrations on wafer processing.
Stepper matching generally refers to the process of determining which steppers work well together, i.e., matching steppers such that when two layers are printed using different steppers, there is a minimum overlay error between the two layers. As should be appreciated, every stepper has its own unique signature of aberrations or other errors and therefore each stepper tends to print patterns differently for a given set of process conditions. The steppers that print patterns in a similar manner are matched to minimize the impact of these aberrations and other errors over the entire process. In most cases, stepper matching is performed by providing a golden wafer having a standard pattern; printing patterns on the golden wafer with each stepper using the same reticle and processing conditions; and calculating the relative difference between each of the steppers by comparing the alignment between the standard pattern and each of the stepper patterns. If the alignment between steppers is similar, then the steppers tend to work well together. If the alignment between steppers is different, then the steppers may not work well together. Although stepper matching for optical aberrations of the lithographic lens system provides some benefit, it is not ideal, since it does not provide feedback about the wavelengths of the illumination sources of the various steppers.
Calibrating the absolute emission wavelength of an excimer laser is described in U.S. Pat. No. 6,160,831 to Kleinschmidt. However, within different steppers the illumination source may have different means for focusing, projecting, and amplifying the light, for example, mirrors, light filters, and/or a condenser lens system. This will give different spectral “signatures” for different steppers; for example, the intensity distribution and spread of the light will differ. The different spectral signatures, will also affect which steppers work well together. A change in the spread of the illumination source wavelength and the intensity over the spread may also cause the image to be out of focus, also known as image “smear.” Therefore, merely calibrating the absolute wavelength of an excimer laser within an illumination source will not match the spectral signatures of different steppers.
Accordingly, in order to improve the quality of patterns transferred to photoresists using photolithography, a need exists for a device and system suitable for measuring and comparing the wavelength characteristics of illumination sources of different steppers.